They are interested in the structure of the ACDS (Acetyl-CoA Decarbonylase Synthase) multienzyme complex which is responsible for the synthesis and cleavage of acetyl-CoA in methanogens. To determine the detailed oligomeric structure of the complex (~1 .6-2 M[)a), accurate molecular masses are needed. ACDS complexes from Methanosarcina therniophila and M bUNIT contain five different subunits designated ~, ~, y, 6, and E. The complex can also be dissociated into three protein subcomponents, one of which is (~ + e). It is thought that the (~+E) particle is a heterotetramer (~2E2) of approximately 220 k')a, but gel filtration data have indicated a value of 350 k')a. To start this project, freeze-dried specimens of the (~+E) particle were prepared. Preliminary mass measurements indicate that it is a heterotetramer.